Soil carbon is mostly grass-derived in tropical savannas, even under woody encroachment

Tropical savannas have been increasingly targeted for carbon (C) sequestration from afforestation, assuming large gains in soil organic C (SOC) with increasing tree cover. Because savanna SOC is also derived from grasses, this assumption may not reflect real changes in SOC under afforestation, but grass contributions to SOC and changes in SOC with increasing tree cover remain poorly synthesized. Here, we combine a case study from Kruger National Park, South Africa, with data synthesized from tropical savannas globally to show that grass-derived C constitutes more than half of total SOC to a soil depth of 1-meter, even in soils directly under trees. The largest SOC concentrations were associated with the largest grass contributions (> 70% of total SOC). Regionally and across the tropics, SOC concentration was not explained by tree cover. Both SOC gain and loss were observed following increasing tree cover, and on average SOC storage within 1-meter profile only increased by a negligibl..., SOC dynamics across Kruger: We extensively sampled soil cores across a semiarid savanna in Kruger National Park to explore factors that influence C4-derived C (i.e., grass-derived C) and SOC concentration at the regional scale.  Kruger covers nearly 20,000 km2 (lat. 22°20'-25°30'S, long. 31°10'-32°00'E) and consists of tropical and subtropical savannas. Mean annual rainfall increases from 350 mm in the north to 750 mm in the south with most precipitation falling from November to April. Elevation ranges from 260 to 839 m above sea level. Based on long-term fire records, the average fire return interval for Kruger is c. 3.5 years, but fire regimes vary spatially across the park, ranging from one fire per year to one every 34 years. Kruger is dominated by two underlying parent materials, granite and basalt, which strongly influence soil and vegetation properties. The flora of Kruger includes over 400 woody species and 200 herbaceous species. The herbaceous layer is dominated by the C4 gram..., The dataset comprises a CSV file with different tabs showing soil organic carbon measurements from Kruger National Park and more broadly across tropical and subtropical savannas.